An atomizer that utilizes a piezoelectric vibrator has conventionally bee for many purposes. Sometimes, an atomizer is desired to be portable. In a portable atomizer, since a battery is used as a power supply, low power consumption is required in addition to usability.
Further, since such portable atomizer tends to be used with various situations (postures/positions), a load applied to a piezoelectric vibrator will be frequently changed. Therefore, it is preferable to provide a measure for maintaining vibrations of the piezoelectric vibrator constant with respect to a varying load. And, it is also preferable to provide a measure for absorbing individual variations in vibrator resonance frequency, load resistance, etc. in mass production.
Moreover, in such portable device, size reduction may also be a challenge. Thus, it is preferable to simultaneously archive size reduction of a battery through the reduction in the power consumption, size reduction of components and reduction in the number of components. For example, while a boosting section for obtaining a high voltage for driving of a piezoelectric vibrator is needed, when a DC-DC converter or a transformer is used as in a conventional technique, a large component will be necessary, and power loss is caused in a boosting section. Thus, it is also preferable that a piezoelectric vibrator is driven by the lowest possible boosted voltage and that a boosting circuit is simplified as much as possible while maintaining a high overall system efficiency.
Generally, driving modes for a piezoelectric vibrator are divided into a forced driving system (separate excitation system) and a self-excitation vibration system. In the forced driving system, an external signal is applied to a piezoelectric vibrator to forcedly drive the piezoelectric vibrator. In this system, although activation is fast and usability is high, a frequency of the external signal is not directly relevant to a resonance frequency of the piezoelectric vibrator, and therefore, power required for driving tends to increase. Thus, the forced driving system unsuitable may not be suitable for a portable device.
In the self-excitation vibration system, an oscillation circuit in which a piezoelectric vibrator is used as a resonator is formed. The piezoelectric vibrator is vibrated by its resonance frequency. While this system is advantageous that loss is small and a reduction in power consumption is enabled, takes time for the piezoelectric vibrator to be sufficiently vibrated. Thus, the self-excitation vibration system is disadvantageous in usability.
Therefore, there is proposed an alternative system (feedback system) which is basically a forced driving system having a self-excitation oscillator but is provided with a feedback circuit so that an oscillation frequency of the self-excitation oscillator becomes a resonance frequency of the piezoelectric vibrator.
JP-H01-293170-A, JP-H05-023646-A, JP-H05-212331-A and JP-H06-063507-A each disclose a technique related to the feedback system for allowing a driving frequency to follow “resonance frequency” of a piezoelectric vibrator. Note that, in these techniques, the “resonance frequency” does not mean the resonance frequency of an LC series resonance circuit of the piezoelectric vibrator, but means the resonance frequency of the entire piezoelectric vibrator, including a damping capacitor. These techniques are broadly classified into:
TECHNIQUE 1 in which a driving frequency is controlled so that a current flowing through an entire piezoelectric vibrator is maximized; and
TECHNIQUE 2 in which a driving frequency is controlled so that a phase of a current flowing through an entire piezoelectric vibrator and that of a driving voltage coincide with each other.
FIG. 2 illustrates an equivalent circuit Sx′ of a piezoelectric vibrator Sx. As illustrated in the equivalent circuit Sx′ in FIG. 2, the piezoelectric vibrator Sx is represented as the equivalent circuit Sx′ configured so that a damping capacitor Cc is connected in parallel to a series circuit of an equivalent capacitor Cx, an equivalent inductor Lx and an equivalent resistor Rx. The series circuit of the equivalent capacitor Cx, the equivalent inductor Lx and the equivalent resistor Rx forms an LC series resonance circuit. When the piezoelectric vibrator is vibrated to cause atomization of liquid, a mechanical load is applied to the piezoelectric vibrator, and the applied load is represented as a increase in the value of the equivalent resistor Rx in the equivalent circuit Sx′.
FIGS. 5A and 5B each illustrates frequency characteristics obtained when a driving voltage Ve is applied to the equivalent circuit illustrated in FIG. 2. A current flowing through the LC series resonance circuit is represented by Ix and will hereinafter be referred to as a “series resonance current Ix”, a current flowing through the damping capacitor Co is represented by Iy and will hereinafter be referred to as a “damping current Iy”, and a current flowing through the entire piezoelectric vibrator is represented by Iz and will hereinafter be referred to as a “driving current Iz”, Px, Py and Pz represent phases of the respective currents with respect to the applied voltage. The phases are expressed in degrees and represented by the scale on the right side of the drawings.
FIG. 5A illustrates a case where the value of the equivalent resistor Rx is low, and FIG. 5B illustrates a case where the load is applied and the value of the equivalent resistor Rx is increased. In both of FIGS. 5A and 5B, the amplitude of the applied driving voltage Ve is the same, and the same current scale and the same phase scale are used.
A frequency fx is a series resonance frequency where the equivalent capacitor Cx and the equivalent inductor Lx are brought into a series resonance state, the resulting impedance becomes 0, and the series resonance current Ix flowing therethrough becomes completely resistive. Therefore, the resulting phase Px coincides with the phase of the driving voltage Ve, and maximum power is supplied to the equivalent resistor. Accordingly, a driving frequency will be adjusted to the series resonance frequency fx in order to supply the maximum power with respect to the load at the same driving voltage Ve. While the series resonance frequency fx depends on temperature, it does not change depending on the load.
In the state of FIG. 5A, in the above-mentioned TECHNIQUE 1, the piezoelectric vibrator is driven by a driving frequency fz1 by which the driving current Iz is maximized. On the other hand, in the above-mentioned TECHNIQUE 2, the piezoelectric vibrator is driven by a driving frequency fz2 by which the phase Pz of the driving current Iz with respect to the driving voltage Ve becomes 0. As understood from FIG. 5A, the driving frequencies fz1 and fz2 are both deviated from the series resonance frequency fx by which the series resonance current Ix is maximized.
In the state of FIG. 5B, in the above-mentioned TECHNIQUE 1, the driving frequency fz1 is obtained, but as understood from the comparison with FIG. 5A, a difference between the driving frequency fz1 and the series resonance frequency fx is increased. On the other hand, in the above-mentioned TECHNIQUE 2, there exists no frequency by which the phase Pz of the driving current Iz becomes 0, and therefore, the driving frequency is uncontrollable.
The above results are caused due to the fact the driving current Iz flowing through the piezoelectric vibrator is provided by a combination of the series resonance current Ix and the damping current Iy. The damping current Iy is capacitive and thus has a leading phase with respect to the driving voltage Ve, and therefore, the driving current Iz also has a leading phase with respect to the driving voltage Ve. Further, the frequency fz1 by which the driving current Iz is maximized is lower than the series resonance frequency fx.
And, when a load change occurs, the current flowing through the equivalent resistor Rx, i.e., the series resonance current Ix, is also changed, and the magnitude of the driving voltage Ve is changed in an actual circuit. Even when a control for making the magnitude of the driving voltage Ve or the current constant is performed, since such control is performed based on the driving current Iz (not the series resonance current Ix), effect due to a change in the series resonance current Ix depending on the load could not be eliminated.